1. Field of the Art
The present invention relates to a method of determining an oxygen partial pressure of a measurement gas, and more particularly to a method suitable for reliable determination of whether a measurement exhaust gas is a rich-burned exhaust gas or a lean-burned exhaust gas, and for fast and efficient measurement of an oxygen partial pressure of the exhaust gas.
2. Related Art Statement
There have been known various electrochemical devices, each of which comprises an electrochemical cell using a solid electrolyte body. Such electrochemical devices are used, for example, as oxygen sensors to detect the oxygen concentration of an exhaust gas (combustion exhaust gas) produced by internal combustion engines of automotive vehicles. Typical examples of such oxygen sensors include an oxygen sensor which employs a mass of oxygen-ion conductive solid electrolyte such as zirconia ceramics, to determine the oxygen concentration according to the principle of an oxygen concentration cell.
An oxygen sensor (sensor for detecting the oxygen concentration of a gas) as such known type of electrochemical devices is capable of detecting an electromotive force which is induced between a measuring electrode disposed on the solid electrolyte body and exposed to the exhaust gas, and a reference electrode exposed to a reference gas, due to a difference in oxygen concentration between the exhaust gas and the reference gas. By detecting the electromotive force, the oxygen sensor determines whether the measurement gas, i.e., the exhaust gas is a rich-burned exhaust gas or a lean-burned exhaust gas. The rich-burned exhaust gas is produced during combustion of an air-fuel mixture whose air-fuel (A/F) ratio is lower than the stoichiometric air-fuel ratio at which a stoichiometric exhaust gas is produced. The lean-burned exhaust gas is emitted when the air-fuel ratio of the air-fuel mixture is higher than the stoichiometric air-fuel ratio. Thus, the engine which produces the exhaust gases is controlled based on the determination by the oxygen sensor whether the measurement gas is a rich-burned or a lean-burned exhaust gas.
The above-indicated type of oxygen concentration sensor is adapted to sense the oxygen concentration of the exhaust gases by utilizing a variation in the electromotive force induced where the exhaust gases are produced at air-fuel ratios in the neighborhood of the stoichiometric air-fuel ratio (A/F=14.6), namely, by utilizing the so-called ".lambda. curve". Therefore, the application of the known oxygen concentration sensor is limited to those exhaust gases which are emitted where the air-fuel ratio of the air-fuel mixture is around the stochiometric ratio. The above-indicated types of electrochemical sensors were originally developed and designed as oxygen sensors for sensing a measurement gas such as an exhaust gas which is produced as a result of combustion of an air-fuel mixture at the stoichiometric air-fuel ratio. Recently, however, it has been proposed to use such oxygen sensors as so-called "lean A/F" sensors for sensing a lean-burned exhaust gas which is emitted in combustion of a fuel-lean air-fuel mixture, i.e., as a result of combustion with an excessive amount of air. Namely, the "lean A/F" sensor is capable of detecting the oxygen partial pressure of an oxygen-rich exhaust has whose oxygen partial pressure is higher than that of the stoichiometric air-fuel ratio. In recent years, however, there have been proposed oxygen concentration sensors which are provided with diffusion-resistance means in various forms such as a porous layer and a pin hole or aperture, that provides a predetermined diffusion resistance to the molecules of a component in the measurement gas. Such diffusion-resistance means are provided to widen the application range of the measurement gas.
In an oxygen concentration sensor provided with such diffusion-resistance means, a given component such as oxygen in the measurement gas is introduced into the sensor for contact with its measuring electrode, at a relatively low partial pressure of the component. Hence, this type of oxygen sensor is capable of dealing with not only a lean-burned exhaust gas whose oxygen partial pressure is higher than that of the stochiometric A/F ratio, in other words, which is produced in combustion of an air-rich or fuel-lean air-fuel mixture whose A/F ratio is far higher than 14.6, but also a rich-burned exhaust gas whose oxygen partial pressure is lower than that of the stoichiometric A/F ratio and which contains a large amount of unburned components, that is, which is produced in combustion of a fuel-rich air-fuel mixture whose A/F ratio is lower than 14.6.